1. Field of the Invention
The present invention relates to a vehicle identification method and device, and in particular to a vehicle identification method and device which recognize a vehicle number from a number plate of a vehicle whose image is taken.
2. Description of the Related Art
A vehicle identification device which recognizes a vehicle number by taking an image of a number plate of a vehicle and performing image processing has been conventionally known. The vehicle identification device has been applied to a vehicle management system for preventing e.g. an unauthorized pass of ETC in an expressway and an unauthorized usage of a parking lot.
In the vehicle identification device, a video type camera performing an interlaced scanning such as an NTSC system is generally used. In the interlaced scanning, a single frame of an image is composed of two fields, i.e. an odd field and an even field. However, since there is a capturing time lag ( 1/60 sec.) in the fields, a vehicle which is an identification object becomes blurred in an image of a single frame.
Therefore, in the prior art vehicle identification device, character recognition processing of a number plate has been performed by using an image of only one field composing a single frame and an image of the other field has not been used.
Also, in the prior art vehicle identification device, near-infrared rays or the like have been used for irradiation (hereinafter, referred to as LED irradiation) at the time of a light quantity shortage such as in the nighttime, in a rainy day, or in a cloudy weather, and then images of vehicles have been taken.
An arrangement and an operation of such a prior art vehicle identification device will now be described referring to FIGS. 6 and 7.
A vehicle identification device 100 shown in FIG. 6 is composed of an image-taking portion 20 which is a camera taking images of a vehicle 10, an irradiating portion 30 which performs the LED irradiation at the time of the light quantity shortage and an image processor 40 which captures an image from the image-taking portion 20 to perform the image processing.
The image processor 40 is composed of a comparison portion 41 which compares an image captured from the image-taking portion 20 at step S10 with an image captured last time to confirm whether or not a vehicle which is a recognition object exists, an image extractor 42 which extracts a number plate area when the recognition object vehicle exists and a character recognition portion 43 which recognizes a vehicle number from the number plate area extracted.
FIG. 7 shows an operation flow of the image processing by the image processor 40 in FIG. 6. At step S100 corresponding to step S10 of FIG. 6, an image is captured. It is supposed that an odd field image F1 is captured and processed. The processing procedure will now be described.
At step S110, the comparison portion 41 compares an image captured last time with an image captured this time, and confirms whether or not the identification object vehicle exists. Namely, in the presence of an identification object at step S120, the image extractor 42 performs number plate area extraction processing (at step S130), and the character recognition portion 43 performs the character recognition processing for the number plate area extracted (at step S140).
Based on the result of the character recognition processing at step S140, whether or not a character has been recognized is determined at step S150. When it has been recognized, a vehicle identification is completed (at step S900).
When it is determined that there is no identification object at step S120, an unnecessary image is discarded because of absence of an identification object at step S160. Also, when it is determined that a character can not be recognized at step S150, the process returns to the image capturing processing of step S100, regarding it as the vehicle identification failure (at step S170).
It is to be noted that at step S100 of FIG. 7, the processing is started at predetermined intervals. In case of the interlaced scanning of the NTSC system, the processing is started at intervals of 1/30 sec.
In the above-mentioned prior art vehicle identification device, the number plate area is extracted from the taken image, and a character part and a background part are specified by binaryization, thereby performing the character recognition.
As such a prior art vehicle identification device, a device which reliably detects a driving vehicle from an image with a comparative low resolution taken by a single camera and which reads a number plate with a high accuracy from the detected image is provided (see e.g. patent document 1).
(Patent Document 1)
Japanese Patent Application Laid-open No.2001-273461 (Abstract, FIG. 1)
As for a number plate presently used, there are a normal one and a backlit one. In the prior art vehicle identification device, as mentioned above, such a difference of kinds of number plates is not taken into account when the irradiating portion 30 performs the LED irradiation at the light quantity shortage.
For example, FIG. 8A showing an image in a case where a number plate whose image is taken by the image-taking portion 20 while the irradiating portion 30 performs the LED irradiation is a normal number plate is compared with FIG. 8B showing an image in a case where a number plate is a backlit number plate. In the latter case, a light quantity emitted by the backlit number plate itself is added to the light quantity by the LED irradiation. Therefore, it is recognized that when the image taken in an excessive light quantity is binarized, the outline of the character of the number plate is blurred.
In this case, the problem occurs that a character recognition for the backlit number plate can not be performed.